Which means the "75 MP" sensor is really a 25 MP sensor in terms of spatial resolution.

Its true only in terms of sensor architecture. In terms of image spacial resolution situation is far more complicated. While bayer and anti-aliasing filter leads to image spacial resolution loss in bayer sensors, its scattering of red photons in foveon. The reality is somewhere halfway for each. At the least, 25 MP foveon is going to be far superior to 25 MP bayer sensor in terms of spatial resolution of image.

What I am wondering is what current-technology equivalent mp value would a 75 megaphotosite sensor maps to in terms of resolved detail?

Theoretically thats dependent on what you shoot - gray gets picked up by all sites and you get the full resolution. Pure colored red fabrics for example otoh are a nightmare, only a quarter of the sites and repeating patterns that can cause artifacts.Practically: Even at ISO100 whether a 5D3 or D800 give you more usable resolution depends on about everything but the sensor... 25MP RGB and a sync speed that avoids motion blur and cuts ambient light enough to avoid narrow apertures/shortens flash duration plus a Canons current glass/AF? Sounds great!

What I am wondering is what current-technology equivalent mp value would a 75 megaphotosite sensor maps to in terms of resolved detail?

Hard to say because there is no single measure or resolution and therefore, no way to compare different sensors based on a single number.

I think line pairs per millimeter is pretty standard and directly comparable.

At what loss of contrast (MTF)? Measured in what way: slanted edge or something else? If so, how slanted is the slanted edge? Demosaiced or not? At what channel (color)? Vertical, horizontal resolution, or something else?

DXO used to publish much more "diversified" data. The numbers/curves depended a lot on the color, for example, not to mention on the MTF value.

At what loss of contrast (MTF)? Measured in what way: slanted edge or something else? If so, how slanted is the slanted edge? Demosaiced or not? At what channel (color)? Vertical, horizontal resolution, or something else?

DXO used to publish much more "diversified" data. The numbers/curves depended a lot on the color, for example, not to mention on the MTF value.

I think MTF 50 is fairly standard. As to what color channel, the final demosaiced RGB image of a black and white subject is fine. Color artifacts from demosaicing affects contrast, so that is taken into account.

I think you are WAY over thinking what I was asking. I will rephrase. "Asking semi-rhetorically, I wonder what megapixel class of bayer filter camera this new camera will compare to in terms of line pairs per millimeter at MTF 50. 30-ish? 40-ish? 50-ish?"

At what loss of contrast (MTF)? Measured in what way: slanted edge or something else? If so, how slanted is the slanted edge? Demosaiced or not? At what channel (color)? Vertical, horizontal resolution, or something else?

DXO used to publish much more "diversified" data. The numbers/curves depended a lot on the color, for example, not to mention on the MTF value.

I think MTF 50 is fairly standard. As to what color channel, the final demosaiced RGB image of a black and white subject is fine. Color artifacts from demosaicing affects contrast, so that is taken into account.

I think you are WAY over thinking what I was asking. I will rephrase. "Asking semi-rhetorically, I wonder what megapixel class of bayer filter camera this new camera will compare to in terms of line pairs per millimeter at MTF 50. 30-ish? 40-ish? 50-ish?"

Until recently, DXO considered MTF 10 to be "the standard", measured on the G channel, at 5 degrees slope, not demosaiced. So you see that their definition of what was standard differed a lot from yours. As a result, there was very clear jump in resolution, from, say 12m to 15mp, then to 18mp, same lens. Then they decided to change the metric. Now, a 36mp sensor does not look that different from a 21/22mp one. The new (secret) metric seems to emphasize more higher MTF values, and might react differently to, say, aliasing; or the angle.